Speleothems of Inazumi Cave, Oita, submerged by the big eruptions of Mt. Aso

Abstract

A submerged calcite speleothem sample from Inazumi Cave, Oita, was dated by TIMS. The speleothem became submerged by accumulation of the Aso welded-tuff. The age of the analysed portion of the sample was 170,000 years B.P., suggesting that it was deposited before the earlier Aso-2 and & -3 eruptions and has been drowned in the groundwater at least three times. Many speleothems are submerged for a period longer than over 85,000 years, and therefore the groundwater had been nearly at dissolution equilibrium with respect to calcite due to slow water circulation. However, undersaturation with calcite dissolution has sometimes happened to the groundwater after the cave was opened for public in 1976 by artificial lowering of the water level by about 4 m.